1. Field of the Invention
The present invention generally relates to an information recording and/or reproducing apparatus for recording or reproducing information on or from an optical disc, respectively. More specifically, the present invention relates to a double-sided disc recording and/or reproducing apparatus for recording or reproducing information on or from a double-sided optical disc, respectively.
2. Description of the Prior Art
The optical disc memory is currently highlighted as one of high-density, non-contact information storage media. Because of a high memory capacity and a capability of use in a non-contact information recording and/or reproduction, the optical disc is generally recognized superior to a magnetic recording and/or reproducing disc. So long as a media size of 5.25 inches is concerned, the double-sided magnetic disc having a maximum memory capacity of 1.5-GB and the double-sided optical disc having a maximum memory capacity of 1.0-GB are currently available.
However, most of the conventional optical disc recording and/or reproducing apparatus (hereinafter referred to as an optical disc player) is equipped with a single optical head and, therefore, the maximum on-line memory capacity to which the single optical head can make an access without the optical disc being reversed is generally limited to half the maximum memory capacity, that is, 500-MB. A relatively recent development is an optical disc player equipped with two optical heads one allocated for each side of the double-sided optical disc, an example of which is schematically shown in FIG. 6. It is to be noted that, in the system of FIG. 6, a servo circuit and a retrieval circuit are not illustrated because they are well known to those skilled in the art.
Referring to FIG. 6, the double-sided optical disc generally identified by 1 comprises a pair of disc-shaped substrates 40 having respective recording surfaces 3A and 3B (which surfaces 3A and 3B may be referred to as disc side A and disc side B of the double-sided optical disc as a whole), said substrates 40 being concentrically bonded together by means of a bond layer 38 with the recording surfaces 3A and 3B confronting with each other. The prior art double-sided optical disc player shown therein generally comprises a pair of optical heads 4A and 4B for projecting respective laser beams towards the disc sides A and B of the optical disc 1, respectively, and a pair of data recording and/or reproducing circuits (Data Rec/Rep CKT) 36A and 36B associated respectively with the optical heads 4A and 4B. Each of the data recording and/or reproducing circuit 36A and 36B is designed to apply a recording signal, i.e., a signal to be recorded onto the optical disc 1, to the associated optical head 4A or 4B when the system is in a recording mode, or to demodulate a recorded signal, i.e., a signal picked up from the optical disc 1, supplied from the associated optical disc 4A or 4B when the system is in a playback mode. Reference numerals 37A and 37B represent respective data lines through which data to be recorded or demodulated data are supplied to or from the associated data recording and/or reproducing circuits 36A and 36B.
FIGS. 7(a) and 7(b) illustrates the recording surfaces 3A and 3B, i.e., the disc sides A and B of the double-sided optical disc 1, as viewed from the associated optical heads 4A and 4B, respectively. As shown therein, each side A and B of the optical disc 1 has tracks 39A or 39B divided into a plurality of sectors S0, S1, S2, . . . and Sn which is the basic unit of storage. It is to be noted that the spiral direction of the tracks 39A on the recording surface 3A is opposite to that of the tracks 39B on the recording surface 3B.
FIGS. 8(a) and 8(b) illustrate, in timed relationship, signals reproduced by the optical heads 4A and 4B from the recording surfaces 3A and 3B of the double-sided optical disc 1, respectively. In each of these figures, reference character ID represents an identification area for each sector S0, S1, . . . and Sn in which track and sector addresses are recorded.
Each of the optical heads 4A and 4B retraces concentrically the tracks of the same address by jumping the tracks at the sector S0.
The double-sided optical disc 1 is manufactured by preparing two replica discs using two master discs that have respective tracks 39A and 39B thereon extending in the opposite spiral directions, forming a recording layer on one surface of each of the replica discs and bonding one the two replica discs to the other of the replica disc so that, while the one of the replica discs is driven in one direction about its center, the imbalance attains a value lower than a predetermined value when both replica discs are bonded together thereby to secure a dynamic balance. Because of this, the position of the sector S0 on the disc side A does not align with that of the sector S0 on the disc side B in a direction across the thickness of the optical disc 1 and is angularly offset relative thereto with respect to the axis of rotation of the optical disc 1.
The manner in which information on the respective sectors on each of the recording surfaces 3A and 3B is recorded by the associated optical head 4A and 4B shown in FIG. 6 will now be discussed with reference to FIG. 8.
It is assumed that data of a size which would occupy the sectors S1 to Sn are to be recorded on the recording surfaces 3A and 3B, respectively. Then, data fed through the data line 37A are modulated by the data recording and/or reproducing circuit 36A and are then recorded by the optical head 4A on the recording surface 3A to fill up data field areas within the respective sectors S1 to Sn as shown by d1a, d2a, . . . and dna. Similarly, data fed through the data line 37B are modulated by the data recording and/or reproducing circuit 36B and are then recorded by the optical head 4B on the recording surface 3B to fill up data field areas within the respective sectors S1 to Sn as shown by d1b, d2b, . . . and dnb.
The data recorded in the above described manner on the track in each sector can be reproduced in the following manner.
When the optical head 4A scans the sector S1 on the recording surface 3A, the data recording and/or reproducing circuit 36A demodulates the reproduced signal fed from the optical disc 4A and then outputs the data d1a through the data line 37A. As the optical head 4A sequentially scans the remaining sectors S2 to Sn, the data recording and/or reproducing circuit 36A similarly sequentially outputs the respective data d2a to dna through the data line 37A. In a similar manner, the data recording and/or reproducing circuit 36B associated with the optical head 4B sequentially outputs the data d1b to dnb through the data line 37B.
However, since as hereinbefore described the position of the sectors on the recording surface 3A are angularly offset relative to that of the sectors on the recording surface 3B by an angular distance which, in the example shown in FIG. 8, corresponds to about 2 sectors, the complete information recording requires an extra time corresponding to a length of time which would pass subsequent to the completion of information recording on a track component in the last sector Sn on the disc side A and prior to the completion of information recording on a track component in the last sector Sn on the disc side B as indicated by T1. Accordingly, that means that, when it comes to the reproducing the recorded, the optical disc 1 has to be driven an extra time which is essentially equal to the length of time T1 discussed above.
In other words, during the recording mode, the complete information recording may takes a total length of time corresponding to the sum of the time required to complete an actual information recording on a required number of the sectors plus the time during which the optical head scans idle over the offset angular distance. On the other hand, during the playback mode, data output is delayed a length of time corresponding to the time T1 and, therefore, the complete information reading takes a total length of time equal to the time required during the recording mode. Because of this, when it comes to the recording or reproducing of information on or from both disc sides A and B of the double-sided optical disc, the angular displacement in position of the sectors between the opposite sides A and B obviously result in a reduction in throughput.